GTAW is a widely used welding process in which an electric arc is formed by a welding torch, a shroud of inert gas, such as argon, is applied from the torch to the weld area and a weld material, such as titanium or 17-4 PH stainless .Iadd.steel.Iaddend., is supplied as a relatively thin wire. The weld and filler material forms a so-called "puddle" in the weld area, e.g., the space or joint between two pieces. The characteristics of the puddle are dependent on the type of wire, weld material, thickness and heat sink properties. Stainless steel is known to produce a weld bead that has waves or ripples to the eye. Titanium flows more easily, producing a generally smooth weld bead.
The characteristics of the puddle indicate the integrity of the weld in the sense that if the arc is too hot, the puddle will be larger. The temperature of the arc is determined by the arc current. The current heats the work-piece and the size of the puddle is a function of the temperature in the weld area. More current is needed to maintain the work-piece weld area at a given temperature as the work-piece gets thicker, and conversely, if the material thickness decreases, the weld area will get hotter. This can lead to changes in puddle width, indicating the change in the materials thickness. Thus, the weld characteristics will change as the work-piece's thickness changes. Even small changes in the thickness can produce undesired changes in the weld strength, not to mention, that uneven heating can produce local variations in material qualities of the workpiece, because areas that are heated to a higher temperature may undergo slightly more removal of important material components. An ideal weld provides a uniform weld bond and minimal material damage.
Experienced welders know those characteristics and visually inspect the weld puddle during the welding process. In a totally manual process, a welder controls the current input as well as the torch height and the amount of wire. The welder slowly moves the welding torch along the joint manually controlling the time spent, wire feed rate and arc intuitively to achieve what visually appears to be a uniform puddle with the correct dimensions.
Automated GTAW, also in use, attempts to provide more uniform welds by controlling temperature, motion of the head and wire feed rates. For example, U.S. Pat. No. 4,724,302, describes .[.a.]. an automated welding process that maintains certain bead dimensions within specified ranges to achieve proper weld strength. There, the system employs a method of automatically controlling a weld bead (puddle) based on the characteristics of the bead height and width using interpolated values of bead height and width. For the most part, this approach typifies those that employ Boolean logic, one in which a particular object, for example a variable, is or is not a member of a given set of parameters. Traditional systems, in other words, have used rigid "set theory" using crisp values.
Welding is an area of technology that is particularly suited to approaches that do not use rigid Boolean set theory, but instead uses set theory in which a particular parameter has a degree of membership in one or more groups. This approach imitates human thinking and is called "fuzzy logic", in contrast with rigid Boolean logic that is the foundation of tradition computational control systems.
Fuzzy logic was introduced by Zadeh in 1965, and, generally speaking, deals with such inherently fuzzy human concepts as "very, most, few" but as applied in a rigid mathematical framework. This may be called "fuzzy subset theory". The application of fuzzy logic to systems that have used Boolean set theory in the past, has produced the fast growing field of so-called "artificial intelligence". The objective of these systems it to .[.capturing.]. .Iadd.capture .Iaddend.the knowledge of the human expert in a particular problem area, representing it in a modular, expandable structure and transferring it to others. The process involves considering the questions of process physics, knowledge acquisition, knowledge representation, inference mechanism, control strategies, user interfaces and uncertainty. The issue of uncertainty, which human knowledge intuitively always considers, is the essential issue in the design of fuzzy logic systems, and for a simple reason, much of the information in the knowledge base of a human is imprecise, incomplete or not entirely reliable--in other words, uncertain. For instance, in GTAW, the weld quality is the desired end but is reflected in some way, not precisely known or understood, in the puddle dimensions.
The harsh, small and unusual environment in which welding takes place presents vexing problems in effectively applying a fuzzy logic based process control approach. For example, problems arise in sensing or detecting the characteristics of a molten metal puddle in the presence of an electric and an inert gas cloud.